The use of liposomes and other lipid structures, such as micro-emulsions, micelles, and drug/lipid complexes, for drug delivery has been widely proposed. Such lipid structures, and particularly liposomes, have the potential for providing controlled "depot" release of an administered drug over an extended time period, and of reducing the side effects of the drug, by limiting the concentration of free drug in the bloodstream. These advantages apply to a variety of routes of administration, including intravenous, intramuscular, and subcutaneous, application to mucosal tissue, or delivery by inhalation. Where liposomes are administered by intravenous delivery, liposomes provide a further advantage of altering the tissue distribution of the drug. Liposome drug delivery systems have been reviewed (Poznansky, Gregoriadis).
In the case of liposomes, optimal size for use in parenteral administration is generally between about 100 nm and 300 nm. Liposomes in this size range can be sterilized by passage through conventional filters having a particle size discrimination of about 200 nm. This size range of liposomes also favors biodistribution in certain target organs, such as liver, spleen, and bone marrow (Gabizon), and gives more uniform and predictable drug-release rates and stability in the bloodstream. Liposomes whose sizes are less than about 300 nm also show less tendency to agglutinate on storage, and are thus generally safer and less toxic in parenteral use than larger-size liposomes.
It may also be desirable to prepare uniformsize liposomes in a selected size range less than about 100 nm. For example, small unilamellar vesicles (SUVs) having sizes between about 30-80 nm are useful in targeting to tumor tissue or to hepatocyte cells, because of their ability to penetrate the endothelial lining of capillaries. SUVs are also advantageous in ophthalmic liposome formulations, because of the greater optical clarity of the smaller liposomes.
Many techniques for preparing liposomes and other lipid structures have been proposed (e.g., Szoka 1983). Typically, prior art liposome preparation methods yield liposomes which are heterodisperse, and predominantly greater than about 1 micron (1,000 nm) in size. These initial heterodisperse suspensions can be reduced in size and size distribution by a number of known methods. One size-processing method which is suitable for large-scale production is homogenization. Here an initial heterodisperse liposome preparation is pumped under high pressure through a small orifice or reaction chamber. The suspension is usually cycled through the reaction chamber until a desired average size of liposome particles is achieved. A limitation of this method is that the liposome size distribution is typically quite broad and variable, depending on a number of process variables, such as pressure, number of homogenization cycles, and internal temperature. Also, the processed fluid tends to pick up metal and oil contaminants from the homogenizer pump, and may be further contaminated by residual chemical agents used to sterilize the pump seals.
Sonication, or ultrasonic irradiation, is another method that is used for reducing liposome sizes by shearing, and especially for preparing SUVs. The processing capacity of this method is quite limited, since long-term sonication of relatively small volumes is required. Also, localized heat build-up during sonication can lead to peroxidative damage to the lipids, and sonic probes shed titanium particles which are potentially quite toxic in vivo.
A third general size-processing method known in the prior art is based on liposome extrusion through uniform pore-size polycarbonate membranes (Szoka 1978). This procedure has advantages over homogenization and sonication methods in that several membrane pore sizes are available for producing liposomes in different selected size ranges. In addition, the size distribution of the liposomes can be made quite narrow, particularly by cycling the material through the selected-size filter several times. Nonetheless, the membrane extrusion method has limitations in large-scale processing, including problems of membrane clogging, membrane fragility, and relatively slow throughput
Co-owned U.S. Pat. No. 4,737,323 for "Liposome Extrusion Method" describes a liposome sizing method in which heterogeneous-size liposomes are sized by extrusion through an asymmetric ceramic filter. This method allows greater throughput rates, and avoids problems of clogging since high extrusion pressure and reverse-direction flow can be employed. However, like the membrane extrusion method, the filter-extrusion method requires post-liposome formation sizing. Further, the method may be limited where uniform-size SUVs are desired.
One limitation of all of the above-mentioned methods is the loss of encapsulated material as large liposomes are broken and reformed as smaller vesicles.
In none of the liposome-preparation methods mentioned above are liposomes with a narrow, substantially symmetrical size distribution produced.